Planning to build a homebrew collinear for both mobile and base station use, but I'm confused about grounding requirements. I've read about adding an extra 1/4 wave coaxial section with opposite phase to put the entire antenna at the same DC potential, connecting to a lightning spike at the top. Since collinear antennas are hot with RF along the coax shield, I understand ferrite toroids are necessary to prevent RF from coming back through the feedline. For mobile use, does the vehicle ground plane eliminate the need for the DC grounding section? My initial test shows 1.2:1 SWR on 146 MHz and under 1.5:1 on 440, with repeaters 60-75 miles away now full quieting.

Diamond dual-banders like the X200/X500 use series-fed 5/8 sections for UHF with additional phasing coils to also work on 2m. For mobile, the vehicle body provides your ground reference - no need for the lightning protection section. Remember tuning can be accomplished with the toroids, and SWR will be lower once mounted in the air.

Those are excellent SWR numbers! Return loss should be greater than 10 dB minimum, preferably above 15 dB. These antennas handle the worst weather - the original WA6SVT design has performed admirably on Santiago Peak at 5670 feet for many seasons. How many elements are you using?

Your buddy's right about the clamp possibly affecting SWR - use another piece of PVC below the connector to mount to the pole. Be careful with paint choices - some contain lead or chemicals that could affect performance or attack the plastic.